The need for reliable and accurate testing of communication lines and for distinguishing between problems existing on a subscriber side of a demarcation point and the telephone company on the opposite side of the demarcation point in a cost effective manner has become of ever increasing importance.
In published PCT patent application, International Publication No. WO 94/13090, published Jun. 9, 1994, apparatus and method for performing a diagnostic test cycle on a communication line from a central office is disclosed comprising a verification controller (VC) located at the central office and respective network termination units, each being located at the locus of a subscriber on the other side of a demarcation point between the telephone company and the respective subscriber. The verification controller is programmed to effect measurements in a timed relationship with an actuation sequence of switches in the network termination unit. Each network termination unit is connected in series with the tip and ring conductors and includes a switch module which establishes a network of electrical paths capable of altering the electrical condition of the communication line, a logic module controlling the operation of the switch module and a power module that accumulates an electric charge from voltage impressed across the tip and ring conductors of the line in order to power the logic module when the network termination unit is actuated. The power supply module also generates a clock signal at a predetermined frequency which regulates the operation of the logic module. The switch module includes a plurality of discrete switches comprising a tip switch for placement in series with the tip conductor, a ring switch for placement in series with the ring conductor, a tip to ground switch, a ring to ground switch, a short switch connected across the tip and ring conductors and a termination switch incorporating a known fixed impedance connected across the tip and ring conductors. Each switch is disclosed as a solid state semiconductor device which establishes a bi-stable electric path capable of assuming either a short circuit condition or an open circuit condition in response to an actuation signal generated by the logic module.
Although the network termination unit made in accordance with the above patent application is effective and accurate, it is larger in size and more expensive in cost than desirable for widespread application.